gh-59705: Add _thread.set_name() function

Lib/test/test_threading.py

@@ -2117,6 +2117,10 @@ def test_set_name(self):
             # test short non-ASCII name
             "namé€",
 
+            # embedded null character: name is truncated
+            # at the first null character
+            "embed\0null",
+
             # Test long ASCII names (not truncated)
             "x" * limit,
 
@@ -2142,6 +2146,8 @@ def work():
 
         for name in tests:
             encoded = name.encode(encoding, "replace")
+            if b'\0' in encoded:
+                encoded = encoded.split(b'\0', 1)[0]
             if truncate is not None:
                 encoded = encoded[:truncate]
             if sys.platform.startswith("solaris"):

Modules/_threadmodule.c

@@ -2421,21 +2421,22 @@ _thread_set_name_impl(PyObject *module, PyObject *name_obj)
         return NULL;
     }
 
-    const char *name = PyBytes_AS_STRING(name_encoded);
 #ifdef PYTHREAD_NAME_MAXLEN
     // Truncate to PYTHREAD_NAME_MAXLEN bytes + the NUL byte if needed
-    size_t len = strlen(name);
+    size_t len = PyBytes_GET_SIZE(name_encoded);
     if (len > PYTHREAD_NAME_MAXLEN) {
-        PyObject *truncated = PyBytes_FromStringAndSize(name, PYTHREAD_NAME_MAXLEN);
+        PyObject *truncated;
+        truncated = PyBytes_FromStringAndSize(PyBytes_AS_STRING(name_encoded),
+                                              PYTHREAD_NAME_MAXLEN);
         if (truncated == NULL) {
             Py_DECREF(name_encoded);
             return NULL;
         }
         Py_SETREF(name_encoded, truncated);
-        name = PyBytes_AS_STRING(name_encoded);
     }
 #endif
 
+    const char *name = PyBytes_AS_STRING(name_encoded);
 #ifdef __APPLE__
     int rc = pthread_setname_np(name);
 #else